Armor plate of aluminum alloys has become established for specialized purposes where not only ballistic resistance, but also lightweight are important considerations. This is notably true in the case of armored military personnel carriers which operate on the ground but must be transportable by air. U.S. military specifications have been developed for such alloys, dealing with ballistic performance in terms of the speeds of two different kinds of projectiles fired at specified obliquities to the target. One of these is an armor piercing projectile (e.g., .30 caliber) designated "AP", characterized by a pointed leading end. The other is a fragment simulating projectile (e.g., 20 mm) designated "FS", characterized by a blunt leading end. The latter projectile tends to create flying fragments from the inner side of the armor plate, even when the projectile fails to penetrate the plate, so that speeds less than penetration speeds have to be considered for purposes of FS tests.
Experience shows that an armor alloy better than another for one kind of these projectiles may be worse for the other kind of projectile. Weldability (joining characteristics and joint performance) and corrosion resistance, which are also important considerations, may also vary for different alloys. Consequently, the general objective is to develop armor plate alloys having improved performance in dealing with both kinds of projectiles, while also achieving good weldability and corrosion resistance.
The aluminum armor alloys which have become most widely accepted are 5083 meeting the requirements of U.S. Military Specification MIL-A46027F (MR), and 7039 meeting the requirements of U.S. Military Specification MIL-A46063E. Alloy 5456 is listed in the former specification, but apparently has had little, if any, acceptance for armor plate purposes. These and all other four digit alloy designations herein are in accordance with alloy numbers and corresponding definitions registered by The Aluminum Association, Washington, D.C.
As shown in these military specifications, armor plate of alloy 7039 is considerably superior to armor plate of alloy 5083 for AP ballistic performance, but less so in FS ballistic performance. In fact, below 1.235 inch gauge, 7039 armor plate is rated below 5083 armor plate in FS ballistic performance, according to the military specifications. In any case, the generally favorable ballistic performance of 7039 armor plate is seriously offset by the fact that it is more susceptible to stress corrosion than 5083 armor plate, especially when welded into an armored structure. It is also less readily weldable than 5083 armor plate, and is more dense than 5083 armor plate, due to the relatively high magnesium and low zinc content of 5083.
Accordingly, there has remained a need for an improved aluminum-based armor plate which attains the best of the AP and FS ballistic performances of 7039 and 5083 plate while also attaining the good qualities of 5083 plate as regards corrosion resistance, weldability and light weight.